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[TowerTalk] Cable loss revisted.

 To: [TowerTalk] Cable loss revisted. "Jim Thomson" "Tower and HF antenna construction topics." Fri, 5 Feb 2010 05:34:31 -0800 mailto:towertalk@contesting.com>
 ``` I visited some contest stations, and after having a look at things, cringe at the thought of the loss on the coax cables used. Some had really long runs of coax from station to base of tower.. then more up the tower, etc. 300-400' was typ. Biggest cables used were 1/2" heliax. A lot of it was 213-U Some was LMR-400 or eq. I don't see why one would want to try and squeak out another .5 db of gain on a longer boom yagi, [ then repeat on every other yagi].. then turn around and lose a whole bunch from the long run's of coax being used !! Sure... 1db doesn't sound like much.. but that's 21% of ur power gone.... and it drives me nuts to think I burned up 21% of the power coming out of the linear. That equates to 472 watts burned up. [ 2250w pep out x .21 = 472 w] Here in Canada, we are allowed 2250 w pep out on SSB.,,,....but only 750W on CW. [ and 750 W CXR on AM] Power is measured ... 'across a Z matched load' . Well the load IS the feed point of the ant. We all interpret Z matched load as 'forward pwr - rvs power' . W0XB's wattmeter will even do the math's for you.... ['net power'] The end result ? Now I have to crank out 2250/.789 = 2851 watts pep ..... just to get my 2250w pep at the ant feed point. And that's with JUST 1 db of loss ! IMO... investing in lower loss coax is the real answer. TX/RX signals lost in the coax, are lost forever. Either that, and/or... put the 10m yagi's / upper band stuff close to the shack... and the low band stuff out in the back 40. Sure you can use on on line calculator's and various schemes to measure coax loss. Humour me.. and haul a bird /CD meter up the tower.. and get someone to stuff exactly 1kw of cxr into the coax in the shack. Then measure at the top of the feedpoint, or close to it... like the top of tower / remote switch box / stack matcher etc. For the folks with 1.5 kw....... 1 db loss means you only end up with 1171 watts at the feed point. So you are no where near 1.5 kw. Stick 1.5 kw into the coax... and see what you actually get. The results will sometimes make you gag. I had moisture get into a connector... and didn't realize it.... till I measured 50 k ohms across the coax... in the shack [ gamma matched yagi yrs ago] Removed the coax seal glop off the pl-259 at feedpoint of 20m yagi.. and water came out! I had foolishly not used any vapor wrap + /88/33 tape. If I hadn't of measured the dc resistance of the coax on a hunch, [should have read infinite ohms] I would have kept on going.. with lousy performance. I took a hint from W2PV... and all cables are now well documented. I measure everything, including Capacitance between center conductor and braid, dc resistance, calibrated watt meter reading at far ends [ on at least 80-20-10m], initial install date, when coax was initially purchased, from who, and how much \$\$, which brand connector's used, exact water proof method employed, etc. You can laugh, but 6 yrs down the road, when you suspect something is amiss, you now have a bench mark /reference.. to compare to. Ditto for all control cables too. later...... Jim VE7RF _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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